c++/modules: Add testcase for fixed issue with usings [PR115798]

This issue was fixed by r15-2003-gd6bf4b1c932211, but seems worth adding
to the testsuite.

	PR c++/115798

gcc/testsuite/ChangeLog:

	* g++.dg/modules/using-26_a.C: New test.
	* g++.dg/modules/using-26_b.C: New test.
	* g++.dg/modules/using-26_c.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>

gcc/testsuite/g++.dg/modules/using-26_a.C

@@ -0,0 +1,15 @@
+// PR c++/115798
+// { dg-additional-options "-fmodules-ts" }
+// { dg-module-cmi base }
+
+module;
+#include <cstdint>
+export module base;
+
+export {
+  using ::int8_t;
+}
+
+export namespace std {
+  using std::int8_t;
+}

gcc/testsuite/g++.dg/modules/using-26_b.C

@@ -0,0 +1,10 @@
+// PR c++/115798
+// { dg-additional-options "-fmodules-ts" }
+// { dg-module-cmi xstd }
+
+export module xstd;
+import base;
+
+export namespace std {
+  using std::int8_t;
+}

gcc/testsuite/g++.dg/modules/using-26_c.C

@@ -0,0 +1,9 @@
+// PR c++/115798
+// { dg-additional-options "-fmodules-ts" }
+
+import xstd;
+import base;
+
+int main() {
+  static_assert(__is_same(int8_t, std::int8_t));
+}